1. Field of the Invention
The invention relates generally to the field of hydrocarbon drilling system control. More specifically, the invention relates to optimized performance of various drilling operations based on downhole measurements.
2. Background Art
The drilling of oilwells is controlled by the judgment and direct human actions of the driller operating the mechanical and electrical systems of the drilling rig. The driller will typically directly control at the surface control station, for example, drill pipe speed and position, the vertical force applied to drillstring, the rotary speed of the drillstring and the flowrate of the drilling fluid. These parameters, among others, may be controlled within limits such as the physical limitations of the rig equipment, or in some cases, pre-defined limits of the input or output parameter, e.g. the torque applied to the drillstring can be limited. The drillers choice of parameters is the result of his general understanding of the feedback responses he gets from the surface equipment, and general observation. This is imperfect information since it does not typically include direct information about the downhole behavior of the drillstring, the formations being drilled or to be drilled, and their relation to the input parameters at surface and the resulting consequences and efficiencies.
On older rigs the control of the drilling parameters is purely manual and relies solely on the driller. New surface drilling control systems are now available which can be programmed to execute an instruction or series of instructions. At present these automated surface control systems are used to control various drilling process segments, for example, such as making a pipe connection. Further, present surface equipment control systems provide that limits be set on certain drilling parameters. However, the limits or values are again a matter of judgment and tend to be a single value per operation per parameter, typically predefined at the initiation of a drilling sequence and without modification or optimization during the drilling process.
Existing controls on the drilling operations provided to a drill operator in many cases restrict maximum efficiency, at least due to the fact that the limit calculations are merely forecasts of the expected drilling properties and earth formations. For this reason, the operations limits, typically provided in absolute parameter values such as an actual rpm, are heavily diluted with error margins. Further, the limits have been developed to generically apply to the entire depth of a borehole, and are not dependent on the specific formation properties encountered.
Approaches have been attempted to refine the limits based on substantial changes to the drilling process. However, even this effort is typically left to human initiative. Thus, to the extent operating guidelines can be modified during the drilling process, substantial risks of human error are introduced into sensitive drilling operations. For this reason, most modifications to drilling processes have been left to the experience of the drilling operator. However, a drill operator's capability to perform certain analyses is limited both by time (limited time to perform testing and calculations) and human ability (limited to relatively simple comparisons). Further, even when a manual analysis is made, the process of implementing a modification introduces error in part due to the drill operator matching to absolute parameter values, many times using analog instrumentation. These limitations in turn introduce inconsistent drilling practices as new drilling operators rotate across work shifts.
To assist in minimizing drilling operation inconsistency, charts have been developed which provide points of reference for some of the drilling parameters. For example, a chart may list a range of drill rpms and a range of downward bit weights to determine an adequate mud flow rate. However, these charts, like the original drilling operations limits, are calculated well in advance of the actual drilling and are thus based on predictions of the drilling conditions. Further, a basic limitation of the charts is due to the inherent finite restriction of the discrete data points, requiring the operator to interpolate between the available data points to fit the actual conditions in order to deduce the proper drilling modification.